Switched local area networks use a network switch for supplying data frames between network nodes such as network stations, routers, etc., where each network node is connected to the network switch by a media. The switched local area network architecture uses a media access control (MAC) layer enabling a network interface to access the media. The network switch passes data frames received from a transmitting node to a destination node based on the header information in the received data frame.
A network switch such as a workgroup switch typically includes port buffering at both input and output buffers. Specifically, a non-blocking switch typically includes ports having input buffers and output buffers such as first in first out (FIFO) buffers, that are sized to accommodate the transfer of data between a source and destination port at wire speed. However, congestion of an output buffer may occur if multiple data packets from multiple input buffers are directed to a single output buffer. Hence, an output buffer may be unable to keep up with reception of data packets from multiple input buffers within the switch.
Flow control has been proposed to reduce network congestion, where a transmitting node temporarily suspends transmission of data packets. A proposed flow control arrangement for a full-duplex environment, referred to as IEEE 802.3x[2] specifies generation of a flow control message, for example a PAUSE frame. A transmitting station that receives the PAUSE frame enters a PAUSE state in which no frames are sent on the network for a time interval specified in the PAUSE frame. Flow control may be implemented on a half-duplex medium by "backpressure," where a receiving node forces a collision so that the transmitting station will abort, backoff, and retransmit.
If flow control is implemented in a switch, however, the transmission of data packets from the respective network nodes is effectively halted until the output congestion eases. One problem associated with implementing flow control in full-duplex links is head of line (HOL) blocking, where a transmitting node sending data packets to the workgroup switch may be forced into a PAUSE interval, even though the transmitting node is attempting to send data packets to a destination node via a network switch port other than the congested network switch port. In addition, outputting flow control PAUSE frames to all the network nodes may unnecessarily reduce all network traffic while the congested output buffer is outputting the data frames. Hence, generation of PAUSE control frames may substantially reduce the throughput of the network unnecessarily.
Similar problems are encountered in a half-duplex environment where a switch uses backpressure. If a particular output buffer is deemed congested, then when a packet arrives at the input buffer of the network switch, the destination address of the packet is decoded; if the destination port is the congested port, then a collision is induced during reception of the data packet to cause the end station to abort transmission, back off, and retransmit. The backpressure is used as long as the output buffer is congested, and hence halts all traffic on the shared medium.